Magnetic Resonance Imaging (MRI) apparatus rely on differences in induced magnetization in biological tissues as a source of image contrast. The induced magnetization can induce a current in a receiver coil(s). The signal measured in a receiver coil(s) is the integral of the induced magnetization over an imaged volume. The signal measured in a receiver coil(s) can be characterized by, for example, a signal to noise ratio (SNR). A certain SNR may be required to allow faithful reconstruction of an image and/or spectra from a magnetic resonance (MR) signal.
To reconstruct an original three dimensional (3D) distribution of magnetization induced in an object, a spatially varying magnetic field pattern (gradient) is generated in the volume so that voxels at different locations produce a signal having spatially distinguishable (e.g., spatially encoded) information. Phase and frequency can be controlled in individual voxels so that each voxel can be distinguished by the phase and frequency of the signal it produces. However, a sampling method that produces an inadequate SNR may make it infeasible and/or impossible to reconstruct an image from the acquired signal.
Image quality may be characterized by factors including, but not limited to, resolution and SNR. SNR is determined by the amount of measured magnetization relative to, for example, thermal noise in an image. The magnetization available to measure depends on the power and duration of an RF (radio frequency) pulse(s) applied to a sample, on the time interval between excitation pulses, and so on. Traditional approaches to improving SNR include using stronger fields, increasing TR (repetition time), averaging over shots, and so on. However, some of these parameters are limited by physical factors (e.g., gradient slew rate, field strength).